It is well known in the prior art to provide a field shunt operation for a transit vehicle electric motor. The motor commonly operates from zero speed up to an intermediate base speed for motoring or power mode with a full field operation to provide a desired torque in relation to the acceleration request P signal, and then when the back EMF voltage of the motor builds up as a function of operating speed, it becomes necessary to shunt the field for a weakened field operation to enable more armature current for holding the desired torque for operation at greater speed above that intermediate base speed. When the field shunt operation is desired, since motor torque is proportional to the field flux times the armature current in order to provide the same desired output torque, an increase in the armature current is required since the field flux is now decreased.
It is known in the prior art to control the operation of one or more transit vehicle motors with a chopper apparatus, as disclosed in U.S. Pat. Nos. 3,559,009 of J. M. Mills, 3,543,121 of L. G. Miller and 3,530,503 of H. C. Appelo et al., and in a publication entitled "Alternative Systems for Rapid Transit Propulsion and Electrical Braking" in the March 1973 Westinghouse Engineer at pages 34 to 41. It is also known in the prior art to provide a full field and a shunt field operation of a traction motor for the control of dynamic braking as disclosed in U.S. Pat. No. 3,569,811.
The typical chopper control apparatus for a transit vehicle motor requires a commutation circuit for determining the effective duty cycle, and this chopper apparatus has a current limit established by the power supply line voltage available to commutate the chopper by the commutation circuit. Frequently, this current limit restricts the required increase in armature current when a field shunt operation is undertaken to result in not holding the desired motor torque when a field shunt operation is made and to result in a jerk in the vehicle motor operation. This jerk is not desired in relation to vehicle passenger comfort.
A general description of microprocessors and the related peripheral devices suitable for the practice of the present invention is provided in the Intel 8080 Microcomputer Systems Users Manual currently available from Intel Corp., Santa Clara, California 95051.